Groundwater quality is increasing in importance and assessing it requires understanding flow and transport processes under unsaturated conditions. Because the physics of unsaturated flow is a highly non-linear function of porosity, permeability, and water retention characteristics (Zhang and van Genuchten 1994), relatively small changes in these properties, induced from chemical reactions, can significantly alter the hydraulic characteristics of a porous medium. Despite this acknowledged significance of reaction-induced changes in hydraulic properties of porous and fractured media (e.g. rocks), little experimental work has been done to quantify the processes involved; most of the work has relied on modeling simulations only. For example, Ortoleva et al. (1987) and Chen et al. (1990) have simulated coupled flow and reaction processes related to advanced oil recovery. Hoefner and Fogler (1988), Sanford and Konikow (1989), Steefel and Lasagna (1990), and Steefel and Lichtner (1994) have simulated porosity/permeability and transport property changes in response to dissolution/precipitation reactions. Smoot and Sagar (1990) inferred from model simulations that the hydraulic conductivity of the soil underneath a waste storage tank at Hanford was halved by leaking hyperalkane liquid.
Using a constant head permeameter, Goldenberg, Margaritz, and Mandel (1983), and Raffensperger and Ferrel (1991) showed how the permeability of sand/clay mixtures changes in response to the infiltration of NaCl and CaCl.sub.2 solutions under saturated conditions.
Columns and/or flow cells filled with particulate, eg soil, have been known and used for years for measuring particulate parameters, for example hydraulic conductivity and diffusivity both for saturated and unsaturated particulate/soil. The book METHODS OF SOIL ANALYSIS, Arnold Klute, Editor, 1986, section 28-5 shows a flow cell system for steady-state measurement of hydraulic conductivity of unsaturated soils (FIG. 1). The soil 100 is held in a column 102 between an upper plate 104 and a lower plate 106. In operation, the system is used for steady-state determination of hydraulic conductivity with a time-invariant one-dimensional flow of the liquid phase at a given water content. Disadvantages of this system include (1) only a time averaged value of conductivity is obtained, (2) only soil with porosity less than a "bubble pressure plate porosity" can be tested, and (3) only pressure and/or temperature conditions that are within the operating limit of the bubble pressure plate are useable/testable. In other words, instantaneous value of conductivity is not possible, nor are measurements requiring gas pressure greater than the bubble pressure of the upper plate 104 possible using this prior art system.
Another common column is the Wierenga column (FIG. 2) useful for measuring hydraulic conductivity of a particulate (soil) sample under unsaturated conditions. Similar in construction with the flow cell, a column body 200 has a base 202 bubble pressure plate 204 with an effluent drain port 206 through the base 202. A tensiometer port 208 with a tensiometer bubble pressure plate 210 and tensiometer access tube 212 is used to measure water tension. The upper portion of the column is the mirror image of the base including a bubble pressure plate and a tensiometer port. Fluid is introduced into the column through a influent port which is identical in construction to the drain port. Flow through a Wierenga column is achieved by creating a hydraulic head pressure and using gravity as the driving force. Unsaturated conditions within the column are achieved by resticting flow and drawing a low pressure or vacuum on the effluent drain port 206. The upper limit of pressure (as vacuum) that may be imposed upon the Wierenga column is determined by the rating of the bubble pressure plates and the column's construction. As designed, these columns typically have an operational limit of -0.137 bar. Again, the Wierenga column has similar disadvantages as the flow cell described above. Both the flow cell and Wierenga columns are designed for steady-state measurements of hydraulic conductivity or water retention characteristics of soils at room temperature. It is assumed in these tests that water percolating through the apparatus is not altering the test material either physically or chemically.
Prior art columns designed for hydraulic property measurements are intended to provide a single data point at a specific hydraulic steady state that is achieved as rapidly as possible. Reactions, including chemical reactions between the fluid and the porous material, are not monitored nor considered. Water is passed through the column containing a porous material in a single step and weighing the effluent or weighing the filled column or both is all that has been required to obtain the hydraulic data of interest. Because the onset of a dissolution or precipitation reaction is unpredictable, taking days or months of exposure to a fluid under reacting conditions, and because once a change in porosity even begins, the change can come to completion quickly, it would not be possible to observe the change in porosity event and any transient effect upon flow with prior art columns. Any change of hydraulic condition of the porous material during operation of a prior art column would simply be averaged in the final result.
Hence, there remains a need in the art of measuring hydraulic characteristics of soils or other test material for an apparatus and method for dynamically measuring hydraulic characteristics wherein a reaction (physical or chemical) alters the hydraulic characteristics of the porous material.